The invention herein relates to processes for expanding perlite.
Perlite is a mineral of volcanic origin which generally falls into the rhyolitic class. The unique feature of perlite is that it contains several percent of water of hydration. If the perlite is rapidly heated to a temperature on the order of 1600.degree. F. (870.degree. C.) the water is converted to steam and the perlite "pops", i.e. it rapidly expands to a much lower density. The amount of expansion is usually on the order of 4 to 20 times the original volume and the final density of the expanded perlite granules will normally be in the range of about 3.5 to 5 lbs/ft.sup.3 (0.06 to 0.08 g/cm.sup.3) for use as insulating fillers or about 7 to 15 lbs/ft.sup.3 (0.11 to 0.24 g/cm.sup.3) for plaster aggregate use.
Expanded perlite is commonly formed in an expansion chamber. In conventional practice, this chamber comprises a vertical vessel, usually cylindrical, having ports at both ends. At the bottom end is a full premix burner which creates a short intense flame and a short high temperature flame zone just above the inlet port. The burner operates in the presence of large quantities of excess air, since the inlet port is open to the atmosphere. The combustion gases and excess air rise through the vertical expansion chamber and exit through the outlet port at the top. Flow rate of the combustion gases and excess air is maintained at a high rate by the use of high volume fans in the exhaust duct from the outlet port. Not only are there large quantities of excess air, but combustion takes place in the presence of continuously varying quantities of excess air, since there is no compensation for combustion air temperature variations at the exhaust fans and no means to adjust for air leaks in the exhaust lines above the furnace.
The raw perlite ore is normally injected into the vertical expansion chamber at a point somewhat above the short flame zone at the bottom of the chamber. The raw ore falls vertically through the chamber until it reaches the flame zone where it is rapidly heated and pops. The volume expansion and density reduction upon popping is sufficient such that the expanded perlite granules are thereafter buoyant enough to be entrained in the stream of combustion gases and excess air and are thereupon rapidly drawn out of the expansion chamber by the high speed exhaust gas flow. The expanded perlite granules are commonly separated from the exhaust gas flow by separation means such as cyclones.
This type of operation of perlite expanders, which has been commonplace and universal for more than 25 years, is beset with numerous problems:
(1) This process is extremely wasteful of thermal energy. Much of the heat of the flame in the flame zone goes to heat the excess air which is drawn through the inlet port. Since this air merely helps to convey the expanded perlite and does not participate in the expansion process at all, the heat imparted to it is simply wasted. (2) The wall of the combustion chamber contributes to the inefficient use of thermal energy. The wall of a perlite expansion chamber cannot be insulated with conventional insulation techniques because its inner surface then becomes hot enough to melt any expanded perlite granules which come in contact with it. This soon creates a layer of sintered perlite on the inside of the expansion chamber, particularly on the walls of that (usually conical) portion of the chamber immediately above the burner. Chunks of this sintered material frequently are dislodged from the wall and fall into the flame and extinguish it. Consequently the past practice has been to allow sufficient heat loss through the wall of the expansion chamber to keep the inner surface at a low enough temperature such that perlite sintering does not occur. This of course represents a waste of heat.
(3) The use of a conventional premix burner creates a major "hot spot" near the bottom of the expansion chamber. The rest of the expansion chamber must therefore be operated at reduced temperatures such that the hot spot can be maintained at a temperature low enough to prevent excessive sintering and melting of the perlite.
(4) The presence of the conventional premix burner hot spot at the bottom of the expansion chamber results in steep temperature gradients throughout the length of the chamber, and particularly in the regions immediately surrounding the hot spot. The resulting thermal stresses on the wall of the continuously operated vertical expansion chamber are sufficiently great that normally within two to four years of service the wall is so drastically warped out of shape that the entire expansion chamber must be replaced.
(5) The expansion characteristics of perlite ore vary widely, both because of natural mineral variations and the variations within and between particle sizes produced during crushing and grading. The conventional process for perlite ore expansion, utilizing a short flame zone and a generally uncontrolled rate of air flow through that zone, exposes ore in the chamber for short time periods at high temperatures and high thermal shock. Only certain grades of ore can be used profitably under these conditions. The rigidity of conventional expansion chamber conditions thus substantially limits the grades of ore which conventional expanders can use. Currently, only those few grades are mined, allowing chamber conditions, in effect, to limit the raw ore supply.
(6) While conventional expansion chamber conditions limit the usable grades of perlite ore, even those grades considered commercially profitable are not themselves problem-free. In some lots of ore, a substantial portion remains unexpanded. Other lots have a tendency to shatter in the conventional high intensity flame zone. Both the unexpanded and the shattered ore increase the bulk density of the collected product, and are lost to the total yield. Furthermore, in many applications of expanded perlite, unexpanded and/or shattered ore is objectionable, and excessive quantities must be removed by further separation operations.
(7) Sintering of perlite ore in conventional perlite expansion units is particularly severe when attempts are made to reduce the ore density to less than 3.5 lbs/ft.sup.3 (0.06 g/cm.sup.3). In order to obtain the lighter densities firing rates and feed rates must be greatly reduced such that the operation at these reduced densities is not considered economically practical.
(8) Because of the high temperature gradients in the conventional expansion chamber, a substantial portion of heat is absorbed by the chamber walls from the flame zone at the chamber bottom. Thus additional energy is needed to maintain the flame zone at the desired temperature.
(9) Waste heat exiting through the expansion chamber wall also creates high ambient temperatures in the vicinity of the expansion unit. This in turn makes it difficult for operators to work efficiently for extended periods in the vicinity of the expander. The excessive space heating can also be deleterious to instruments and other sensitive devices in the vicinity.
(10) The exhaust fans are subject to high abrasion rates from the expanded perlite. They also have widely varying efficienies because of the variations in the uncontrollable air flow rate through the system.
Heretofore attempts to alleviate these difficulties have primarily been directed to a careful selection of the optimum ore grades for expansion, adjustment of operating temperatures and/or feed rates and to a limited extent control of furnace exhaust draft. These have proved to be of only minimal effect, since they are all subject to wide variation and precise control cannot be maintained over extended periods.
In addition, efforts have been made to preheat at least a portion of the combustion air. Calculations indicating the theoretical effects of such preheating have been described in a paper presented in May, 1976, at a meeting of the Perlite Institute in Montreal, Canada. Actual efforts to preheat the combustion air and utilize such preheated air have not been particularly successful, however, because the preheated air has simply been fed to a conventional full premix burner. This has required that the hot air be drawn through a fan to increase the static pressure of the air prior to entering the burner. Since this fan handles hot expanded air, it must be of extremely large volume to compensate for the low density of the hot air and must also be constructed of materials capable of withstanding elevated temperatures of several hundred degrees. In addition, the necessary controls for such fans and premix burner feeds are complex and costly.
It would therefore be of considerable advantage to have a perlite expansion process which provides for the efficient utilization of thermal energy and high efficiency of expansion of the perlite ore.